In recent years, it has been becoming more and more difficult to increase the volume of semiconductor memories through miniaturization alone, because of their structural limitations. In order to overcome this problem, a 3D-stacked memory has been developed that increases volume by stacking semiconductor memories in the vertical direction (i.e., thickness direction). Examples of the 3D-stacked memories include an HBM (High Bandwidth Memory), an HBM2, and the like standardized by the JEDEC (Joint Electron Device Engineering Council).
In the case of the HBM, for example, four core dies each having a DRAM structure are stacked on a base die. Each core die and the base die have through vias (TSV: through silicon via) formed therethrough, with micro bumps electrically coupling the through vias between vertically adjacent dies.
The core dies have the same structure, so that storing data at the same address in each core die results in the location of the stored data being at the physically same position in all the dies. Namely, in the plan view of the HBM as viewed from above, the locations of the data in the four cores are aligned. Data having a length of one word may be divided by four, and the resulting four pieces of data may be stored at the same address in the four core dies, respectively. In such a case, the one-word data is stored in plural locations at the same plan-view position in a distributed manner. Storing one-word data at the same plan-view position gives rise to the problem that soft error caused by a neutron ray has a great impact as will be described below.
A neutron ray included in cosmic rays is not electrically charged, and is thus more likely to reach the surface of the earth than charged particles. As a high-energy neutron ray reaching the surface of the earth hits a silicon atom contained in a semiconductor memory to generate a secondary ion, a large number of electrons are ejected from the silicon atoms lying on the flight path of the ion. These electrons serve to reverse the memory cell charge, thereby causing soft error.
A secondary ion caused by the impact of a neutron ray may be generated in the uppermost core die of an HBM. A length of a path passing through a plurality of core dies differs between the case in which the secondary ion flies in the direction perpendicular to the core dies (i.e., in the thickness direction of the core dies) and the case in which the secondary ion flies at an angle to the core dies.
In the case of the perpendicular-direction flight, the path of the ion passing through each core die is the shortest, and the path of the ion passing between adjacent core dies is also the shortest. It follows that the ion does not lose much of its energy while passing through a core die and reaching the next core die situated immediately below. As a result, the secondary ion generated by the impact of a neutron ray at the uppermost core die may penetrate plural core dies inclusive of the uppermost one, thereby creating a risk that data is destroyed by soft error in the plural core dies.
In the case of the slanting-direction flight, the path of the ion passing through each core die is relatively long, and the path of the ion passing between adjacent core dies is also relatively long. It follows that the ion experiences relatively great energy attenuation while passing through a core die and reaching the next core die situated immediately below. As a result, the secondary ion generated by the impact of a neutron ray at the uppermost core die is less likely to penetrate plural core dies inclusive of the uppermost one, with a less risk that data is destroyed by soft error in the plural core dies.
In the case in which the divided parts of one-word data are stored at the same plan-view position in a plurality of core dies, these stored data parts may all be destroyed at once by an ion flying in the vertical direction. There is thus a possibility that the entirety of one-word data suffers significant damage. In the case where error correction code such as ECC is put in place, one-bit error is correctable. However, errors occurring at two or more bits cannot be corrected. Accordingly, uncorrectable multiple-bit soft errors may occur when an ion caused by a neutron ray flies in the vertical direction as described above.